Sound sources can be situated at any direction on the horizontal plane. A good surround sound system should therefore reproduce sources situated at different directions equally accurately. Commercially available multichannel systems usually employ uneven loudspeaker positions favouring the front direction, and the audio material to be played back over such systems is typically engineered heavily at the post-processing stages so as to provide a good localization and ambience perception. While satisfactory listener experience can be achieved most of the time, the perceptual consistency of the reproduced audio with the actual recording environment cannot be guaranteed and the reproduced sound field reflects the choices of the audio engineer rather than the properties of the actual recording venue.
There exist different audio reproduction systems based on the concept of reconstructing the sound field exactly. Ambisonics and wave-field synthesis (WFS) are two such systems. The former achieves perfect reconstruction only at a narrow listening area. The latter requires significant computational resources and a high number of channels and is thus not feasible in a domestic setting. A multichannel recording and reproduction system was proposed by Johnston and Lam (J. D. Johnston and Y. H. Lam, “Perceptual soundfield reconstruction,” Presented at the AES 109th Convention, Los Angeles, USA, Preprint #2399, 22-25 September 2000) that overcomes these limitations in order to provide a panoramic listening experience to the listener in a wider listening area.
More particularly, the Johnston-Lam array comprised a circularly symmetric microphone array composed of five first-order microphones on the horizontal plane facing outwards and two superdirectional microphones facing up and down. The stated aim of the Johnston-Lam array was to accurately capture interaural cues of binaural hearing. The recorded audio could be played back with a corresponding loudspeaker array consisting of five equispaced loudspeakers on a circle to provide panoramic audio to the listeners. The signals recorded using up and down facing microphones were mixed to signals obtained with the horizontal microphones. The system was reported to provide very realistic spatial perception. In a later patent (U.S. Pat. No. 6,845,163B1 to Johnston and Wagner), the setup was generalised to having odd number of microphones on the horizontal plane. It was also suggested in the patent that the vertical microphones can be omitted from the system without much subjective degeneration in the reproduced sound field.
In the original proposal and also in the subsequent patent the directivity pattern of the individual array elements were selected so as to have a gain of 3 dB below the front direction gain at the look direction of the neighbouring microphone, and a zero at the next non-consecutive channel. For the original proposal which considered five channels on the horizontal plane this requirement corresponded to having a 3 dB decrease at 72° from a microphone axis, and a zero at 144°. The second-order microphone directivity which satisfies these design considerations is given in FIG. 2.
Whilst the Johnston array provides a measure of panoramic audio recording, improvements in panoramic audio recording, and particularly improved localisation, would be desirable.